This invention concerns packaging films and, in particular, packaging films that can be easily opened by tearing but that, at the same time, protect the product contained within the film from dirt, dust, moisture or other contamination. Exemplary uses for such films include, for example, candy wrappers, tampon overwrap, ice cream wrappers, and individual snack food wrappers. It should, however, be understood that the film of the present invention may have many other uses. The packaging film has an optimized modulus level which allows the film to maintain its shape. For example, in the instance of use of the film as a wrapper for hard candy, the film is wrapped around the piece of hard candy and the protruding edges are twisted. Memory and secant modulus are attributes required to enable the film to keep its position and prevent the hard candy from becoming dislodged until forced out of film by an external force.
In response to this demand, biaxially oriented polypropylene (BOPP) films were produced and achieved an important position in the packaging industry, especially in the packaging of foods, owing to their stiffness, strength, clarity, and moisture barrier properties. Currently, this market segment accounts for about 70% of the BOPP films produced. In general, the films are generally multilayer structures produced by the so-called tenter process. In this process, the polymers are melted and homogenized in extruders. The melts are filtered, and brought together in a slot or annular die, where they are extruded to produce a monolayer or multilayer melt film. In slot die extrusion, the melt curtain is cast onto a chill roll, where it crystallizes. In annular die extrusion, the film is quenched with air to form a solid film. The film is subsequently oriented or stretched in the longitudinal and transverse directions. The stretching process is typically a three-stage process, as described, for example, in EP-0 116 457 B1. After being stretched, the film is cooled to room temperature, and trimmed at both edges. If desired, the film is then surface treated. The film is subsequently wound and packaged according to customer specifications. The application-relevant properties of BOPP films are principally determined by the biaxial orientation and the polymer compositions thereof. However, the biaxial orientation production process has many disadvantages. First, the process involves the use of expensive equipment and introduces an additional step into the manufacturing process, thereby resulting in an inefficient and costly production process. Second, due to the stress during processing of the film, the film is highly susceptible to breakdowns in the course of production, for example, tears in the film.
Several prior art films have been developed to replace BOPP films. One such prior art film is manufactured by coating a polymer film with a resin layer including a propylene copolymer, an α-olefin having four or more carbons, and ethylene. The resulting film exhibits inferior tear properties, due to the presence of ethylene and the crystallinity of the polypropylene components. Further, the film necessitates lamination of the resin layer to the polymer film, which further decreases the tearability of the film.
Another prior art BOPP replacement film is manufactured by orienting the film in only one direction. However, this process involves the use of outdated film orientation equipment, and although it eliminates one processing step, the remaining orientation step is still time-consuming, expensive, and increases the frequency of processing breakdown.
An additional prior art BOPP replacement film involves the addition of a weakening line in the film or scoring the film itself. Alternatively, a row of notches may be added at each end seal of the wrap. However, these packaging films are relatively expensive to produce because of the additional equipment costs and the additional manufacturing steps. Further, where the tear lines are beyond the enclosed product, a great deal of wrap material is wasted, thereby increasing processing costs.
Yet another prior art packaging film involves the addition of a tearstrip. The tearstrip is a narrow band of supportive material adhered along the inside periphery of the wrapped product packaging and ending in a tab that is notched on either side. As the tab is lifted and pulled, due to the notching and pull force, a tear begins to propagate in the direction of the pull. However, the addition of the tab is a costly alternative.
It would be desirable, therefore, to provide a cost-effective packaging film with good stiffness, low tensile elongation at yield, good dead-fold characteristics, and good tearability.